Add support for private DNS with DNS-over-HTTPS (port 443/5053)

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Add support for private DNS with DNS-over-HTTPS (port 443/5053)

Sources:

• https://discourse.pi-hole.net/t/dns-over-https/3814
• https://blog.davelindon.me/dns-over-https/
• https://scotthelme.co.uk/securing-dns-across-all-of-my-devices-with-pihole-dns-over-https-1-1-1-1/
• https://docs.pi-hole.net/guides/dns-over-https/

Pros over DNS-over-TLS:

• Can implement now
• Production ready code

Cons over DNS-over-TLS:

• Not an official standard
• Limited to Cloudflare (they do not block malicious domains as Quad9 does)

DNS-over-HTTPS ticket: https://github.com/teusink/Home-Security-by-Pi/issues/4

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DoH added: https://github.com/teusink/Home-Security-by-Pi/blob/master/2-Configuration.md#pi-hole